The idea was that a thing could only move into an empty place, and that, in a plenum, there are no empty places.

2001, Edward Grant, God and Reason in the Middle Ages, p. 176:

The key to understanding medieval interpretations of motion in hypothetically void space is to realize that medieval natural philosophers analyzed the same bodies in the void that they discussed in the plenum of their ordinary world.

He lay on the long stone slant down to the slapping waves, his denim shorts, sneakers, and socks under his head for a pillow, feeling the splendour of distance in all directions, the liquid silence, the plenum of aloneness.